As mobile devices have been increasingly developed, and the demand of such mobile devices has increased, the demand of secondary batteries has also sharply increased as an energy source for the mobile devices. Also, much research on batteries satisfying various needs has been carried out.
Especially, a lithium secondary battery has higher voltage and more excellent charge/discharge cycle characteristics than a conventional nickel-cadmium battery or a conventional nickel-metal hydride battery, and therefore, the demand of such a lithium secondary battery has sharply increased.
In terms of the shape of the battery, the demand of a rectangular battery or a pouch-shaped battery, which is thin enough to be applied to products, such as mobile phones, is very high. In terms of the material for the battery, the demand of a lithium secondary battery, such as a lithium cobalt polymer battery, having high energy density and high discharge voltage is very high.
The illustrative structure of a general polymer battery is shown in FIG. 1.
Referring to FIG. 1, a battery cell 100 comprises a battery case 110 and electrode terminals 101 and 102 protruding from the battery case 110.
In the battery case 110 is mounted an electrode assembly (not shown) comprising cathodes, anodes, and separators disposed between the cathodes and the anodes. The electrode terminals 101 and 102 are formed by welding electrode taps, which are attached to cathode and anode plates, to electrode leads. The electrode terminals 101 and 102 are partially exposed from the battery case 110. The electrode assembly is mounted in the battery case 110 while the electrode terminals 101 and 102 are partially exposed from the battery case 110, the battery case 110 is filled with a electrolyte, and edges 111 and 112 of upper and lower case parts of the battery case 110 are thermally welded to each other by applying heat and pressure to the edges 111 and 112. In this way, the battery cell 100 is manufactured.
Although the lithium secondary battery with the above-stated construction has more advantages than the conventional nickel-cadmium battery or the conventional nickel-metal hydride battery, the lithium secondary battery has a problem of weakness. Specifically, the electrolyte is injected into the battery case at the back-end process during the manufacture of the battery. For this reason, an organic solvent having low boiling point is generally used. In this case, however, the interior pressure of the battery case is increased when the battery is overcharged or when the battery is left under high temperature. As a result, the external appearance of the battery case may be deformed due to a swelling phenomenon, in which the electrode assembly or the battery case swells, and therefore, the battery case may explode.
In order to solve the above-mentioned problem, there have been proposed methods of hardening a plate-shaped battery using ultraviolet rays or electron beams or coating gel to electrode plates without injecting an electrolyte (U.S. Pat. Nos. 5,972,539 , 5,279,910, and No. 5,437,942). These methods somewhat eliminate the swelling phenomenon, in which the electrode assembly or the battery case swells. However, the above-mentioned methods do not provide satisfactory safety.
Some prior arts propose a method of interrupting the operation of a pouch-shaped battery according to a value detected by a pressure sensor, i.e., strain gauge type sensor, which is attached to the surface of the battery, through a protection circuit disposed between terminals (a cathode and an anode) of the battery and input/output terminals. Specifically, when the battery case, i.e., the pouch, swells, the swelling degree is detected by the sensor, and the detected value is transmitted to the protection circuit, which interrupts the flow of electric current between the cathode and the anode when the detected value exceeds a predetermined level.
However, the method of measuring the swelling degree of the battery case has a limit in providing high reliability, and it is difficult to stably mount the sensor to the surface of the pouch-shaped battery. For example, the battery is minimized due to the reduction in size, weight, and thickness of the battery. Consequently, it is very difficult to accurately measure the swelling degree of the battery case through the surface change of the battery case while maintaining the limited size of the battery case. Furthermore, the strain gauge type sensor needs a large area for accurate measurement. As a result, the heat dissipation of the battery may be disturbed due to the strain gauge type sensor, and therefore, the temperature of the battery may be rather increased. For this reason, the strain gauge type sensor cannot be used in a thin battery, which has small surface swelling.
Consequently, the necessity of a technology to fundamentally solve the above-mentioned problems is highly increased.